1. Field
One embodiment of the invention relates to a magnetic recording apparatus and a magnetic recording medium.
2. Description of the Related Art
The rapid development of information technology (IT) and diverse demands in various information dramatically increase the amount of information produced. Magnetic disk devices are suitable, in terms of capacities, bit costs for storage, access performances, reliability, or the like, for storing therein a large amount of information. In order to increase the capacities of the magnetic disk devices, a recording density is required to be increased. Hence, various technologies for achieving high density recording have been studied and developed.
Increasing a recording density has limitation called trilemma. When the minimum unit of magnetization for recording is reduced to increase the recording density, the recorded information is to be lost after a long period of time because the information recorded with a small unit of recording is thermally unstable. Further, with the use of a recording medium having a magnetic characteristic that is not easily lost, magnetic poles are saturated due to the material characteristics of the magnetic pole of a recording element, thereby results in a state where a sufficient recording magnetic field cannot be output. Still further, when size of the minimum unit of magnetization is increased, the number of units of magnetization each corresponding to one bit of information decreases so as to increase noise (medium noise) in the transition region. As a result, recording and reproduction with sufficient reliability becomes impossible. As described above, the three of a thermal relaxation characteristic, a writing characteristic, and signal quality, are in a trade-off relationship with each other.
The conventional technologies that surmount the limitation on a recording density are, for example, a bit patterned media system, an energy assisted recording system, and a shingle write/two-dimensional magnetic recording (TDMR) system. In the bit patterned media system, one bit of information is recorded in association with a recording land that is physically isolated and provided on a medium. With this system, the volume of the minimum unit of recording can be enlarged and thus the recorded information is less affected by thermal relaxation. The energy assisted recording system is a system by which writing is performed using a magnetic material that is difficult to be magnetically inverted at room temperature by assisting a recording magnetic field with external energy only during recording. For the external energy, thermal energy produced by a laser beam, excitation energy of magnetic spins produced by a microwave, and the like have been studied. With this system, the writing characteristic out of the three factors that inhibits the density from being increased is improved.
In the shingle write/TDMR system, during recording, a number of tracks of recordings each being formed in a movement direction of a recording medium are overlaid with each other with slight offset and recorded in the recording medium to record the information, using a recording element that has a comparatively wide recording width. During reading, magnetic information in a unit of recording/reproduction is read two-dimensionally, using a reading element having comparatively high resolution, and then the information recorded by a two-dimensional signal process is demodulated.
The magnetic recording system as described above is known as a shingle write system or a TDMR system (for example, see The Magnetic Recording Conference (TMRC) 2008, Jul. 31, 2008, Session CC: Recording Physics of Novel Media CC-4 “The feasibility of magnetic recording at 10 Terabits per square Inch on conventional media).
However, in a magnetic recording system disclosed in the conventional technology, clock signals necessary to obtain information by reading one unit of recording/reproducing is not considered. Stable clock signals are required to perform a two-dimensional signal process on the information.